Zhongxiao Zhang

Experimental study on low NOx emission using blast furnace gas reburning and industrial application in stoker boiler

In order to get some useful information on reforming one stoker boiler with the capacity of 75t/h by using gas reburning technology, the effect of some key factors such as stoichiometric coefficient, residence time, reburning temperature and reburning gas - coal heat ratio on NOx emission characteristics was studied in a one-dimensional experimental system. The results showed that the longer residence time of flue gas in reburning zone of furnace will get lower NOx emission, there exists an optimum residence time of flue gas in reburning zone, the optimum residence time is around 0.8s. There also exists an optimum stoichiometric ratio of air in reburning zone of furnace, the value of that is around 1.0. The removing rate of NOx increases with both increasing feeding rate of reburning gas and increasing reburning temperature. The gas reburning technology has been successfully used during reforming an stoker boiler with the capacity of 75t/h, the NOx removing efficiency was over than 50%.

Experimental Study on Low NOx Emission Using Blast Furnace Gas Reburning and Industrial Application in Stoker Boiler

To get the modification method of blast furnace gas reburning technology on one 75t/h stoker boiler, the effect of some key factors such as stoichiometric coefficient, residence time, temperature and heat ratio input on NOx formation regulation was studied on a one-dimensional test system with fuel reburning. The results showed that prolonging the reburn zone residence time can help to lower the NOx emission and the best time was 0.8s. There existed an optimum value 1.0 for reburn zone stoichiometric coefficient. The NOx reduction efficient increased with the increase of the reburn heat input and the reburn zone temperature. The use of blast furnace gas reburning technology has been successfully implemented during the modification of stoker boiler, the NOx reduction rate was over 50%.

Study on the effect of flame offset on water wall tube temperature in 600°C and 700°C ultra-supercritical boiler

ABSTRACT The distribution of water wall temperature in the ultra-supercritical (USC) boilers was obtained by establishing a coupled heat transfer model. The reliability of this model has been validated through the comparisons of simulated and measured water wall temperatures along different dimensions in the furnace of reference USC boiler. Then, the effect of flame offset on water wall tube temperature in 600°C and 700°C USC boilers was investigated by importing the flame offset variables into the coupled model. The water wall temperature distribution is significantly influenced by flame offset, and both the fluctuation and growth rate of temperature are increased with the ascent of flame offset distance, especially on the furnace walls that flame deviates toward. The radiative and convective heat flux to water walls is strengthened simultaneously during the flame offset process, resulting in the local overheating of water wall. In the 600°C USC boilers, when the distance of flame offset exceeds 5 m, multiple peak distribution of wall temperatures appears, which can increase the burst risk of water wall tubes because of shear stress inside the tube material. The maximal distance of flame offset should be limited to 3 m to avoid the tube burst accidents. In the 700°C USC unit, the variation tendency of water wall temperature is resemble with that in 600°C USC unit, but the fluctuation of wall temperature is larger. As the flame offset distance approaches 3 m, the maximal water wall temperature reaches 595°C, which greatly exceeds the material allowable temperature in the 600°C USC unit. The material of water wall tubes with allowable temperature of 605°C is recommended for the 700°C USC unit. Based on the thermal security of metal material, the maximal distance of flame offset should be yielded to 2 m.

Mathematical model on heat flux analysis of water wall in 1000MW USC boiler furnace combustion zone

In order to study the heat transfer between the flue gas and water wall, according to the 1000MW ultra super critical boiler, the flue gas temperature in the furnace, the heat flux in the water wall, the medium temperature in the tube, and ash fouling temperature in different conditions are calculated and compared with the date onsite based on the mathematic model in this paper. The model is composed with the coal combustion and radiation transfer in engineering simplification, and based on the Russia Power Station Boiler Thermodynamic Calculation Method Published in 1998. The results in different excess air coefficient conditions show that the flame temperature in the furnace and the maximum heat flux and ash fouling temperature on the water wall, and application of Air Staged Combustion Technology makes the flame radiation heat transfer distribute evenly and maximum value decrease so that NOx emission reduces and water wall safety is available. This model verified by the plant data can predict the maximum heat transfer position in the heat surface so that the tube overheating and explosion can be avoided which is important for the boiler running.